1. Field of the Invention
The present invention relates to performance grade asphalt and more specifically to a method for making performance grade asphalt.
2. Related Art
Using asphalt for surfacing heavily traveled roads is common practice. Asphalt historically has been a mixture of heavy residual fuel oil obtained from the process of refining crude oil. Often this residual, referred to as asphalt cement and/or asphalt binder by the asphalt industry, may be the residue from the bottom of a crude vacuum tower in a typical petroleum refinery. The residual product is mixed with aggregate (stone, sand, etc.) to produce road asphalt. The mass ratio of asphalt cement to the aggregate is typically 5%. Traditionally, the critical specification of the asphalt cement was its viscosity. The higher the viscosity of the asphalt cement, the more suited the asphalt grade would be for higher ambient temperatures. Asphalt grades were categorized typically as AC-5, AC-10, AC-20, AC-30 with the higher numbers representing the higher viscosity grades, and therefore better suited for higher ambient temperatures such as would be experienced in the southern states. Other specifications which were tested included penetration, ductility, softening point and moisture susceptibility. The asphalt cement, any added modifying materials, and the aggregate are tested individually, and as a finished asphalt blended mix.
With increased use and load demand on asphalt roadways, the U.S. Department of Transportation recognized the need to modify the asphalt specifications and from 1987 through 1993 conducted the Strategic Highway Research Program (SHRP) to develop new ways to specify, test and design asphalt. The final product from SHRP is referred to as "Superpave" or Superior Performing Asphalt Pavements. For discussion purposes, this term is abbreviated as PG, or Performance Grade, asphalt. While PG asphalt testing still examines the viscosity of the asphalt cement or binder, it also analyzes the asphalt over a temperature range. PG asphalt testing artificially ages the asphalt to determine how the material can be expected to perform on the road surface during a several year period. The testing measures void spaces which can result in cracking, compacting or break up. The testing also measures heating losses which can be important when evaluating the effectiveness of asphalt modifiers from both an economic as well as an environmental standpoint. PG asphalt testing also examines and sets standards for the aggregate component of the mixture including such specifications as size (coarse vs. fine), angularity, flatness and elongation, faceted, fractured, etc.
Distinct from the former AC specifications, which graded asphalts according to viscosity, PG asphalt is categorized or graded by the temperature range over which the particular grade will meet or exceed the minimum required specifications. For example, a typical PG grade of asphalt is PG 64-22. This indicates that the asphalt will meet specification throughout a temperature operating range of 64 degrees C. (147 degrees F.) down to -22 degrees C.(7.6 degrees F.). PG asphalt temperature specifications vary in 6 degree increments. For example, the next lower temperature grade below -22 degrees C. is -28 degrees C. The next higher grade above 64 degrees C. is 70 degrees C. Generally speaking if there is less than 90 degrees C. between the upper and lower temperatures in a PG asphalt grade, that grade can more than likely be produced without the need to add performance improving components or modifiers. However, if a specification requires more than 90 degrees C. between the upper and lower temperatures, such as PG 64-28 or PG 58-34, generally either high-end or low-end modifiers are required. The demand for wide range PG asphalt is greater in the northern latitudes as the ambient temperature range is greater there than in the southern areas.
An ideal asphalt is stiff enough to resist compacting or rutting from heavy highway loads during high ambient temperatures, while having a high enough tensile strength to not be brittle under those same heavy loads during very low ambient temperatures. The differential in low ambient temperatures is greater than the differential in the high ambient temperatures between the northern and southern latitudes. Therefore, the ability to drop the lower end of the PG range is more critical than raising the upper end of the range.
Conventionally, the low end of the range is modified through the addition of a petroleum distillate. Initially, asphalt blenders utilized diesel fuel or home heating oil to drop the low end of the performance range. Typical petroleum distillate blended into asphalt cement lowers the upper temperature about two degrees for every one degree that is reduced at the low end of the temperature range. Thus, conventional petroleum distillate reduces the high temperature limit and the low temperature limit of the asphalt cement in a ratio of about 2:1. In order to maintain the upper temperature range when using such conventional distillate fuel oil to lower the low end of the temperature range, it is necessary to add polymer concentrate to counteract the effects that the fuel oil has on the upper end of the temperature range. These polymer concentrates add considerable expense to the overall cost of the asphalt.
Thus, there is a need in the art for an inexpensive performance grade asphalt. Particularly, there is a need in the art for a method of making an inexpensive performance grade asphalt that is effective over a wide temperature range. Still more particularly, there is a need for a method of making Performance Grade asphalt that improves the high to low temperature limit ratio to less than about 2:1.